This is a specific time period, last night. Why the imperfetto and not passo prossimo?
The imperfetto is used to express an habitual action in the past and to describe time, age, and weather in the past.
It can, yes. It just sounds better to say the adjective in front of the word, particularly for day/night. (Like, la prossima notte, l'ultimo giorno, etc.)
Lots of nouns in Italian seem to need the definite article - while it's not needed in the English translation I've been told in these comments before that you'd sound unusual without it
It is he/she/it when the word is used elsewhere. Here, it refers to the weather and the weather is not usually known as he or she.
Correct. Fa caldo or fa freddo is always used in terms of weather to say it's hot or cold.
My translation was: "It was hot last night" and I got bonked. What gives, Duo?